In the automotive industry, exterior metal panels (i.e., closure panels) on vehicles are attached to the vehicle structure during manufacturing using resistance spot welds. Non-metal panels are attached to the metal structure with adhesives or by mechanical fasteners. Subsequent replacement of a metal panel (e.g., due to damage from a collision) is typically accomplished by welding or by a combination of welding and bonding the panel to the metal structure. Thermally responsive bonding materials (e.g., thermally curable adhesives) are utilized for bonding a replacement panel to the metal structure. The cure times of a thermally responsive bonding material can be reduced by applying heat to the bonding material. The bonding materials utilized are typically one-part or two-part adhesives. Such adhesives may be epoxy, urethane, acrylic, or acrylic-epoxy based adhesives.
In the collision repair process, the bonding material is either applied to the replacement panel or to the vehicle structure, or both. The panel is fixed in proper alignment with the vehicle structure. The panels must remain stationary in a heated shop to cure the bonding material until the bonding material has at least developed handling strength. During the time that a collision repair shop waits for the bonding material to cure, the vehicle occupies valuable shop space which could be utilized for other purposes. Different two-part adhesives require varying times to cure adequately to achieve handling strength, and even longer cure times are required for the adhesive to reach its full structural strength. One-part adhesives are not used as frequently in collision repair since one-part adhesives usually require moisture or heat to cure the adhesive. Moisture is known to be slow to penetrate into a thin adhesive bondline sandwiched between a replacement panel and the vehicle structure.
A collision repair shop is required to replace many different sizes and shapes of vehicle closure panels. Known heating apparatuses and methods for accelerating bonding material cure times include infra-red heat lamps, silicon-coated resistant heat tapes, hot-air heat guns, and paint bake booths. Each of the above methods have known disadvantages. Infra-red heaters can provide high-heat to broad areas. However, the high temperature necessary for rapidly curing some bonding materials may also cause damage to unprotected adjacent heat sensitive materials in/on the vehicle. Silicon-coated resistance heat tapes can be taped or clamped along a bondline. As the tape heats, it expands and portions "lift up" from the heated surface. The areas of the bondlines under the raised portions of the heat tape may not receive adequate heat. Paint bake booths can be used for accelerating the cure time of a bonding material, but the whole vehicle occupies a very expensive piece of equipment necessary for curing paints. Some paint bake booths can not be heated to an adequate temperature to cure known structural bonding tapes (SBT) or one-part paste adhesives. If such high temperatures were obtained, the heat could also damage heat sensitive components of the vehicle. Hot air heat guns are able to obtain the temperatures necessary to accelerate the cure of thermal bonding materials. However, curing of a bondline with a point source heater like a heat gun is a very time consuming operation. Only small sections or "spots" of the bondline are heated at a time. In use of a heat gun, it may not be very easy to uniformly control the ultimate bonding material temperature. This may result in overheating of the bonding material to a point of decomposition. Alternatively, inadequate heat could result in an incomplete cure.
Induction heating has been known to be used in the manufacture and assembly of automotive vehicles involving high production rates of similar parts. Electric induction coils are employed to provide heat to accelerate the curing of thermally responsive bonding materials positioned between juxtaposed metal sheets. Such induction coils carry high frequency electrical current which generates a magnetic field and causes heating of the metal sheets. Heat is conducted from the metal sheets to the bonding material disposed between the metal sheets. Known methods of induction heating include the use of spot induction heaters or rigid copper induction applicators. Spot induction heaters concentrate a large amount of heat at a small, localized area or "spot". It is common to employ spot induction heaters at selected locations along the length of a bondline so as to spot cure the bonding material at the locations of the induction coils to achieve handling strength. The remainder of the bonding material is cured at a later time during the assembly process, such as when the automotive vehicle passes through a paint bake booth. One known spot induction heater is disclosed in U.S. Pat. No. 5,442,159 to Shank issued Aug. 15, 1995.
The use of rigid copper induction applicators requires a different shaped applicator for each different shaped bondline or panel geometry. Such rigid induction applicators would not be desirable for use at a collision repair shop, which often requires bondlines of a different panel geometry for each use. Further, due to the high current in the inductor, rigid copper induction applicators often require additional cooling (e.g., a water cooling system) to avoid overheating of the rigid copper induction applicator. One known rigid copper induction applicator is disclosed in U.S. Pat. No. 4,602,139 to Hutton et al. issued on Jul. 22, 1986.